1. Field of the Invention
The present invention relates to an improved gas-phase copolymerization process for producing polymers of propylene and ethylene in multiple reactors using high activity Ti/Al catalyst systems whereby polymerization conditions are controlled through the use of control parameters. By monitoring the copolymer produced, ethylene feeds to either or both reactors and other process conditions are controlled and maintained within a specified range to produce copolymers having improved properties.
2. Description of the Prior Art
Copolymers of propylene and ethylene obtained utilizing Ziegler-Natta catalysts have been described in the prior art. U.S. Pat. No. 4,260,710 describes a process for producing propylene homopolymers and copolymers of propylene with other .alpha.-olefins utilizing Ziegler-Natta catalysts in a stirred vessel using catalyst components which can include a titanium halide and an aluminum alkyl.
U.S. Pat. Nos. 4,454,299 and 4,455,405 describe processes for the preparation of block copolymers of propylene and ethylene in two reaction zones using Ziegler-Natta catalysts. In these processes, propylene is first polymerized in gas form in an initial reaction zone and the resulting homopolymer is then transferred to a second reactor where a mixture of ethylene and propylene are polymerized therewith.
In U.S. Pat. No. 4,547,552 a process is disclosed to produce chemically blended propylene polymer compositions having ethylene contents from 3 to 40 weight percent. The process comprises polymerizing propylene in a first stage with 0 to 5 mol percent of another olefin and, in a second stage, polymerizing propylene and ethylene or propylene, ethylene and another olefin in the presence of the reaction product from the first step. Rubbery propylene copolymer and crystalline ethylene polymer or copolymer are produced in this manner.
U.S. Pat. No. 4,882,380 describes a gas-phase polymerization to produce products having an ethylene-propylene copolymer incorporated in a propylene homopolymer or copolymer matrix. This is accomplished by contacting propylene or another .alpha.-olefin in a first reactor using the prescribed catalyst and then passing the mixture to a second reactor where ethylene and propylene are polymerized to provide an ethylene-propylene copolymer.
Whereas all of the aforementioned processes provide useful polymers, the polymer properties may be deficient in some respects. It would highly advantageous if a process were available whereby polymerization conditions could be controlled to permit broadening the property envelope of the propylene-ethylene copolymers and to eliminate some of the problems associated with heretofore known procedures. These and other advantages are realized with the present improved process.